1. Field of the Invention
The present invention relates to a 2-cycle engine, which includes an exhaust control valve capable of adjusting the opening of an exhaust port so as to perform self-ignition combustion of supply air and supply fuel in a combustion chamber with operation of the exhaust control valve.
2. Related Art
In the 2-cycle engine, a scavenging efficiency drops as the intake quantity becomes small, so that a sufficient exhaust is not performed from the exhaust port to leave the burned gas in a large quantity in a cylinder chamber (although such gas will also be called the “residual gas”). Generally speaking, if the air and the fuel are supplied into the combustion chamber having the burned gas left therein, three components including the air, the fuel and the residual gas are mixed substantially homogeneously in the combustion chamber. The residual gas is heated to a high temperature by the combustion so that it holds a high thermal energy, but has an effect to suppress the combustion because it is an inert gas.
In case the engine is in a low load range, when spark ignition combustion is performed utilizing an ignition plug in a condition where air and fuel supplied into the combustion chamber and burned gas remaining in the combustion chamber at a preceding step are mixed, flame propagation at the time of ignition or after ignition is blocked by the residual gas having the abovementioned effect to cause an irregular combustion. If this irregular combustion occurs, the fuel to be supplied is not completely burned, and the unburned gas containing a harmful substance is discharged to an outside thereby to raise problems in fuel consumption and environments.
On the other hand, there is known (as referred to JP-A-07-071279, for example) an engine provided with an exhaust valve capable of controlling the opening of the exhaust port. This engine is configured in such a manner that: the operation of the exhaust valve is controlled in accordance with a load to adjust the opening of the exhaust port; the ratio of the mass of the residual gas to that of the whole gas in the cylinder at the time of starting a compression stroke (although the ratio will be called the “EGR ratio”) is adjusted; the mixture in the combustion chamber is heated utilizing the thermal energy of the residual gas; and self-ignition combustion of the mixture is performed.
The engine of this structure is configured in such manner that the operations of the exhaust valve and the ignition plug are controlled so that the spark ignition combustion is performed in a high load range and the self-ignition combustion is performed in the low load range. In the self-ignition combustion, since a combustion mode is bulk combustion, the irregular combustion is eliminated entirely in the combustion chamber thereby to improve the fuel consumption and to reduce the quantity of the harmful exhaust gas.
As shown in FIG. 8, the residual gas retains the thermal energy so that the gas temperature in the cylinder chamber at the end of the compression stroke rises as the EGR ratio rises. It has also been confirmed that the self-ignition combustion can be performed if the gas temperature exceeds a predetermined temperature TAR. As shown in FIG. 9, on the other hand, at the end of the combustion stroke, the gas temperature in the cylinder chamber is lowered by the combustion suppression effect of the residual gas itself, as the EGR ratio raises. Even if the actuation of the exhaust valve is controlled to keep the gas temperature at a high value at the end of the compression stroke so that the EGR ratio may rise, the thermal energy for heating the air and the fuel sufficiently at the next compression stroke becomes so short that the temperature TAR necessary for the self-ignition combustion cannot be kept, because the gas temperature at the end of the combustion stroke seriously drops when the EGR ratio exceeds a predetermined EGRAR.
This phenomenon takes place in case, particularly in a high output type engine, the feed of air becomes short in a low load and low rotational speed range so that it becomes difficult to control the scavenging flow. In this area where such self-ignition combustion cannot be performed, the irregular combustion still occurs. The elimination of the irregular combustion in the low load range is a problem.